Organic molecules having large nonlinear polarizabilities have been recognized as potentially useful as components of the optical elements in optical frequency converters and in electrooptic devices. In order to create organic materials exhibiting the large second order optical susceptibilities essential to nonlinear optic applications, the molecules must be constructively arrayed in a noncentrosymmetric configuration. Such molecules have been crystallized in a noncentrosymmetric space group, but this method does not work for all potentially useful molecules and the resulting shape and properties are limited by the very nature of a crystal.
A number of other methods for noncentrosymmetrically arranging the molecules to optimize the nonlinear properties of the resulting organic material have been used. For example, strong DC electric poling fields have been applied to polar dye molecules in semi-fluid polymeric or glassy matrices in order to align the molecules noncentrosymmetrically. The matrices are then rigidified, while still under the influence of the externally applied DC field, to "lock" the at least partially aligned dye molecules in place. In still another approach polar dyes are attached directly to polymeric backbones which are similarly treated to lock the polar dyes in biased alignment. These methods have had limited success, because the polymer matrix allows the molecules therein to "relax" over time thereby losing the configuration necessary for the enhanced nonlinear optic properties. Furthermore, the polymer can dilute the effective nonlinearity, as it is often difficult to get more than 10 to 20 percent of nonlinear molecules into the polymer reagent.
One of the more promising and recent approaches to making stable nonlinear optically active organic materials involves forming highly crosslinked networks where polar molecules (e.g., dyes) are polymerized directly into the polymer reagent matrix during the poling process. Eich et al., J. Appl. Phys. 66(7), Oct. 1, 1989, pp.3241-3247, discloses the preparation of nonlinear optically active crosslinked polymer networks from the reaction of diepoxides, with and without nonlinear optic (NLO) dye moieties, and NLO active di- and tri-functional amines. Hubbard et al., Chemistry of Materials, Volume 1, Number 2, March/April, 1989, pp. 167-169, disclose crosslinked epoxy polymer reagent networks containing dispersed NLO dyes similar to those disclosed by Eich et al. While these epoxide networks provide significantly improved thermal stability of NLO properties relative to earlier polymeric systems, preparation of the epoxide networks require complex processing steps to avoid interruption of the poling process by excessive conductivity.
This invention avoids the processing problems associated with conventional crosslinked polymer networks by forming the network from a mixture of a preformed reactive polymer reagent and a monomeric crosslinking agent, at least part of which contains a dye.